Specific gravity (G) is a measure of wood density affected by the specific species or species combination of the commercial grade, growth region, and grading rules. It is the single most important lumber property for the strength of mechanical fasteners, such as nails and screws used in residential construction. Here, G is the ratio of the weight of oven-dried wood relative to the weight of an equal volume of water. Knowing the volume of a piece of lumber and its moisture content, the weight of a piece of lumber can be easily calculated knowing water weighs 62.4 pounds-per-cubic-ft (lbs./ft3).
The average value of G is published for various species or species group used in residential construction. For example, the G-value for Southern Pine (SP) is 0.55, meaning a sample of oven-dried SP would weigh 55% of the weight of water (62.4 lbs/ft3). Assuming a 2 x 4 x 8 piece of SP, the volume (V) of the piece would be calculated as follows:
V(ft3) = (1.5 in x 3.5 in x 96 in)/1,728 in3/ft3 = 0.29 ft3
Assuming 19% moisture content (MC), the weight (W) of the piece would be given by
W(19%) = (1+MC%) G x V (ft3) x 62.4 lbs/ft3
= 1.19 x 0.55 x 0.29 x 62.4 = 11.8 lbs
The factor 1.19 coverts an oven-dry 0.55 G-value to the maximum MC of 19% for “dry lumber” per the American Softwood Lumber Standard PS 20-20.
Assuming a 2x4x8 Spruce-Pine-Fir (South) at 19% MC, typically denoted on a grade mark by SPF-S, SPFS, or SPFS, would weigh 7.8 lbs (1.19 x 0.36 x 0.29 x 62.4). For softwood framing lumber commonly used in the U.S., the range of published G-values is 0.36 to 0.55. The differences in weight of the various species or specie groups are evident when sorting or handling lumber.
Impact of G on Fastener Strength
To demonstrate the impact of G on fastener strength, the lateral and withdrawal strengths versus G has been illustrated by John Scholten at the U.S. Forest Products Laboratory (Strength of Wood Joints Made with Nails, Staples, or Screws, 1965), as shown in Figure 1. [For Figure, See PDF or View in Full Issue.]
The lateral load strength with a ¾-inch side member is plotted against G at the top of Figure 1. Note that the relationship between lateral strength and G is non-linear, increasing by the exponent 5/4. The withdrawal load of nail connection with 1¾-inch embedment into the main member is plotted on the lower section of Figure 1. Note that the exponent on G is 5/2, dramatically impacting the withdrawal load based on the G-value of the lumber.
For example, the withdrawal strength of the 8d common nail in lumber with a G-value of 0.36 is only 35% of the strength of the nail in lumber having a G-value of 0.55. Knowing these wood technology facts provides incentive for designers to clearly define in their Construction Documents the G-value of the lumber assumed in their structural design work and thus required for the project. What might appear to be a minor difference in a lumber species grade mark may impact the structural performance of a building in a wind or seismic event. Likewise, when selecting lumber for trusses and wall panels, component manufacturers (CMs) should carefully verify that the minimum design values (Fb, Ft, Fc, Fv, Fc-perp, Emin, E, and G) specified in the Construction Documents for the wood framing elements have been met with respect to 2018/2024 NDS Supplement Design Values.
Nail, Lag Screw, and Bolt Design Values
The 2024 National Design Specification for Wood Construction (NDS®) gives design methods and tabulated allowable lateral and withdrawal load values for the full range of mechanical fasteners (carbon and stainless nails, roof sheathing ring shank, lag screws, wood screws, and toe-nails) based on several design properties of lumber, including the G-value of the species. In addition, reference lateral design values for bolted connections are tabulated. Tabulated design values are based on the G-value of the lumber that ranges from 0.35 to 0.67 in most tables, demonstrating the impact of specific gravity on connection strength for the members connected.
Knowing the fastener type and dimensions, details of the connection, and G-value of the lumber, the allowable value for a single fastener can be determined by simple look-up tables in the NDS. Based on the application, the tabulated values are subject to “adjustment factors” given in NDS Chapter 11. The NDS and NDS Supplement are referenced by the ICC building codes (IRC and IBC).
Engineered Fasteners and Connectors Design Values
Fastener and connector manufacturers tabulate the design values for their products based on an assumed species or species combination (hereafter, species). For example, Simpson Strong-Tie (SST) publishes uplift and lateral allowable loads for their connectors (such as the H2.5A) with two sets of design values for “DF/SP” and “SPF/HF.” A table in the SST General Notes information defines the species represented by DF/SP and SPF/HF as well as two lumber design values, compression perpendicular-to-grain (Fc┴) and specific gravity (G), as shown in Table 1. [For table, See PDF or View in Full Issue.]
In effect, by grouping two species combinations, the SST table defines the minimum G-value assumed for the tabulated allowable design values. For species having a G-value of 0.50 or greater, the allowable values in the respective tables with labeled DF/SP apply. For species having a G-value of 0.42 to less than 0.5, the allowable values in the respective tables labeled SPF/HF apply.
SST Important Information and General Notes
Referring to the previous link, footnote “f” to the G-value table states: “Unless otherwise noted, allowable loads are for Douglas Fir-Larch under continuously dry conditions. Allowable loads for other species or conditions must be adjusted according to the code. Allowable loads for alternate species may be determined as outlined in Simpson Strong-Tie engineering letter...”
Footnote “f” alerts the user to the fact that when the designer/user selects a species with a G-value less than 0.42, the tabulated design values for fasteners and connectors must be adjusted to account for the lower G-value.
New Lumber Products Available
Newer grade-marked lumber products that have a different grade mark format from the past are widely available and have published design values in a new NDS Supplement Table 4G. The caption to Table 4G is Reference Design Values for Multi-Species and Country Grademarked Visually Graded Dimension Lumber (2”–4” thick)1,2,3. Prior to 2021, published design values for grade-marked dimension lumber only included one species (such as Douglas fir) or a species group (such as Southern Pine). The unique feature of the grade-marked products in NDS Supplement Table 4G is that one grade mark includes two or more products manufactured in one country, different countries, or regions. For example, see the 2024 NDS Supplement Table 4G, pp. 62–78 for lumber made in the US, Canada, and around the world on the AWC website.
Fastener and Connector Design Value Adjustment Factors
Proprietary fastener and connector design values published by the manufacture must be adjusted for species having a G-value less than 0.42. To assist the designers and other users, some manufacturers have developed and published “adjustment factors” for both fasteners and connectors for lumber having a G-value less than 0.42 as published in the NDS Supplement Tables 4A, 4C, 4F, and 4G. Designers and users should contact the fastener and connector manufacturer for the applicable adjustment factors.
Lumber Substitutions for Walls and Trusses
Lumber products tabulated in NDS Supplement Table 4G are manufactured and grade-marked using the same rules (American Softwood Lumber Standard PS-20) as used in the US and Canada and thus are an additional lumber resource for CMs, homebuilders, and the public. However, the new lumber resources require additional knowledge for interpreting the grade marks correctly, locating the design values for a specific grade mark, and knowing how to make a proper lumber substitution for ordinary wood framing, prefabricated wall panels, and roof and floor trusses. An article in the September 2023 issue of STRUCTURE Magazine, “Robust Lumber Specifications on Construction Documents,” suggests a format for specifying minimum framing lumber properties that include numbers that directly link the grade and species designation to a set of reference design values. As an example for wall framing, the specification might read: “Wall-framing shall be minimum 2×4 No. 2 SPF with minimum reference design values of: Fb/875, Ft/450, Fc/1150, Fc-perp/425, E/1,400,000, and G/0.42.”
As discussed in that article, the properties of the wall framing lumber assumed by the design professional impact their design work related to wind uplift connections and shear wall design. Other issues such as truss and girder bearing capacity can be a design and performance issue as well.
For trusses, the chord and web lumber used for fabrication should have design values that equal or exceed the design values for the grades and species descriptions that are documented by the truss design drawings (TDDs). Being that each lumber product in the NDS Supplement has eight published design values that are used in different combinations for a single truss design, a direct lumber substitution using the NDS Supplement (only) requires one that has eight design values, equal to or greater than, the design values associated with the original design. If a direct one-to-one comparison of design values fails the test, then the truss designer should contact their software provider to obtain the proper design values for the lumber and associated “gripping values.”
Historically, structural designers, fastener and connector manufacturers, and CMs have viewed the strength of lumber in terms of the bending and stiffness values (Fb and E) of a few species groups (SP, DFL, HF, and SPF from Canada) that had G-values of 0.55, 0.50, 0.43, and 0.42, respectively. Designers knew the G-values by memory, design standards and building code tables were based on one of the four G-values, fastener and connectors manufacturers published design values based on the four G-values, and CM truss design software automatically used the plate design values as applicable for the specie/species group selected. With new sources of lumber products available, the G-value of the products (NDS Supplement Table 4G) can be as low as 0.35. Due to the substantial impact of G on strength of mechanical connections (Figure 1 as an example), G-related adjustment factors for fasteners, connectors, truss plates, shear walls, and diaphragms may be required for the published, familiar, and longstanding design values.
Frank Woeste, P.E., is Professor Emeritus, Virginia Tech. Frank, along with his colleagues and peers, has developed and presented Virginia Tech continuing education programs for more than 35 years. He is a member of the AWC Wood Standards Design Committee and serves as the ALSC Consumer Representative (Alternate). Comments are welcome and can be sent to fwoeste@vt.edu.